Comment by masklinn
3 years ago
A compiler can’t know why you fucked up, it can’t even know that you fucked up, because UBs are just ways for it to infer and propagate constraints.
If an optimising C compiler can’t rely on UBs not happening, its potential is severely cut down due to the dearth of useful information provided by C’s type system.
> A compiler can’t know why you fucked up, it can’t even know that you fucked up, because UBs are just ways for it to infer and propagate constraints.
To be honest, that's just how compiler writers interpret UB these days.
It's perfectly possible (in principle) to use lots of more sophisticated static and dynamic analysis to recover much of what C compiler just assume. You don't have to restrict yourself to what C's type system provides.
(For an example of what's possible, have a look at all the great techniques employed to make JavaScript as fast as possible. They have basically no static types to work with at all.)
> For an example of what's possible, have a look at all the great techniques employed to make JavaScript as fast as possible. They have basically no static types to work with at all.
I’m sure people will be very happy with a C JIT. That’s definitely what they use C for.
JIT-ed code is full of runtime type and range assertions which bail if the compiler’s assumptions are incorrect.
Oh, I didn't mean to imply that it would be practical. Only that it's possible and that the type system isn't the only thing you can rely on.
Instead of just assuming that 'x > x + 1' is always true (for signed integers), the compiler could also do the heavy lifting of static analysis (for cases where that's possible).
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